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Astronomers discover oldest stars ever seen

Staff writer |
An international team of astronomers, led researchers from the University of Cambridge and the Australian National University, have identified some of the oldest stars in our galaxy.

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That could contain vital clues about the early Universe, including an indication of how the first stars died.

These stars, which have been at the very centre of the Milky Way for billions of years, contain extremely low amounts of metal: one of the stars is the most metal-poor star yet discovered in the centre of our galaxy.

The stars also contain chemical fingerprints which indicate that the earliest stars may have died in spectacular deaths known as hypernovae, which were ten times more energetic than a regular supernova. The findings, reported today (11 November) in the journal Nature, could aid in understanding just how much the Universe has changed over the past 13.7 billion years.

Using telescopes in Australia and Chile, astronomers may have landed on a winning strategy to find the oldest stars in the galaxy. Stars with a low metal content look slightly bluer than other stars: a key difference that can be used to sift through the millions of stars at the centre of the Milky Way.

Using images taken with the ANU SkyMapper telescope in Australia, the team selected 14,000 promising stars to look at in more detail, with a spectrograph on a bigger telescope. A spectrograph breaks up the light of the star, much like a prism, allowing astronomers to make detailed measurements.

Their best 23 candidates were all very metal-poor, leading the researchers to a larger telescope in the Atacama desert in Chile. From this data the team identified nine stars with a metal content less than one-thousandth of the amount seen in the Sun, including one with one-ten-thousandth the amount - now the record breaker for the most metal-poor star in the centre of the galaxy.

However, knowing that these stars have low amounts of metal wasn't enough to be certain that they formed very early in the Universe. They could be stars that formed much later in other parts of the galaxy that weren't as dense, and they are just now passing through the centre.

To separate those possibilities, researchers measured distances and used precise measurements of the stars' movement in the sky to predict how the stars were moving, and where they had been in the past.

They found that while some stars were just passing through, seven of the stars had spent their entire lives in the very centre of our galaxy. Computer simulations suggest that stars like this must have formed in the very early Universe.

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